Machines where riders could fill up their fare cards had signs attached that said "out of service" and "free Muni" on Saturday. Fare gates and ticket machines were working on Sunday morning.

The attack seems to be an example of ransomware, where a computer system is taken over and the users are locked out until a certain amount of money is sent to the attacker. The Muni hack reportedly included an email address where Muni officials could ask for the key to unlock its systems.

Outages started happening on Friday afternoon but the bulk of the delays started on Saturday, according to a local ABC News affiliate. The extent and the identity of the perpetrator of the hack isn't known yet.

"We are currently working to resolve the situation," a Muni spokesperson told the Examiner. "There is an ongoing investigation and it wouldn't be appropriate to provide additional details."